For, approximately, twenty-five years, certain materials, such as zeolite, vermiculite, perlite, and the like have been expanded into light-weight aggregates. Since 1946, there has been developed apparatus and methods for expanding these certain materials. These materials are given a heat treatment in the range of about 700.degree. F. to about 3000.degree. F. to process them into expanded solid particles. A material which may be used for illustrative purposes is perlite.
Perlite is an aluminal-silicate mineral that is noncrystaline and glasslike in its nature. When perlite ore is ground to the approximate particle size of sand, it has a density of about 75 lbs. per cubic foot, although this density can vary somewhat. Perlite contains a small amount of sodium oxide and potassium oxide which acts as a flux and reduces the melting point of the mixture of the perlite and the oxide. Also, there is a small percentage of chemically combined water in the perlite. When high temperature is applied to a perlite particle, the surface softens and the water turns to steam causing the perlite particle to expand. The average density of the expanded perlite particle is 8 lbs. per cubic foot, although it is possible to get the density as low as 3 or 4 lbs. per cubic foot. The particle size of the expanded product is controlled, to a degree, by the particle size of the ore. The various markets for expanded perlite particles are, essentially, controlled by the particle size of the expanded product.
The perlite ore is mined by open pit methods, either drill and blast, or in some cases, dug, directly, with large bulldozers and rippers. The ore is then transported to a mill where the rock is crushed, ground, dried, and screened to various particle size ranges to meet specifications for various markets. The finished ore is then shipped by covered hopper cars all over the country to expanding plants where it is heated and expanded into its final form and distributed from these plants. The ore presently used comes from five states: New Mexico, Colorado, Arizona, California, and Idaho. Probably, 85% of the ore is mined at No Agua, New Mexico, and shipped from Antonito, Colorado, the closest rail head.
At the present time, the largest particle size of perlite ore used for expansion purposes is about 1/8" in diameter. The largest expanded perlite particle does not exceed about 3/8" in diameter.
The perlite industry received its start toward the end of World War II. In the formative years of the perlite industry, three types of furnaces were developed for expanding the perlite ore to form expanded perlite particles. These three types of furnaces were the stationary horizontal furnace, rotary furnace, and vertical furnace.
The vertical furnace is by far the most popular design. The rotary furnace is next in popularity with the stationary horizontal furnace being third in popularity. The vertical furnace is capable of heating and expanding all gradations of ore. The rotary furnace works best on coarse ore. The horizontal stationary furnace is used on fine ores.
The vertical furnace comprises a vertical tube. A burner is placed at the bottom of the tube looking upwardly and a draft is provided by a fan downstream from the vertical tube. The ore is dropped directly into the flame about midway of the tube. The particles fall down, downwardly, and due to the heat in the vertical tube, the particles are heated and start to expand. With expansion, the density of the particles decreases, and the rate of fall in the vertical furnace slows. Then, when the particles have expanded, sufficiently, the density of the particles decreases and the force of gravity on the particles is overcome by the upward draft in the furnace and the particles reverse their direction and exit out the top of the vertical furnace or vertical tube. These expanded particles are carried, pneumatically, to a collector, such as a cyclone or bag house and collected.
The rotary furnace is, essentially, a set of concentric cylinders that are set, horizontally, and rotate in the same manner as the rotary kiln. There are three cylinders, one inside the other. The ore is fed into the annular space between the inside cylinder and the center cylinder. The ore is preheated in this space. The preheated ore is then fed into the inside cylinder. This inside cylinder has a burner mounted in it that provides the heat. As the ore expands, the lighter expanded particles enter into the airstream passing through the furnace and are carried out of the furnace. The heavier particles are expanded into the expanded particles and put into the airstream at the end of the furnace. The expanded particles are collected in a fashion similar to that with a vertical furnace in that these particles can be collected in a cyclone or bag house.
The horizontal stationary furnace comprises a cylinder and has a burner to supply heat. The fine solid particles are introduced into this cylinder and heated to expand the fine particles. An airstream passes through the horizontal cylinder and the expanded particles, which have a low density, are carried out of the furnace in the airstream and collected in a bag house or a cyclone.
The source of heat or the source of heat energy is a gas, such as natural gas. This gas is burned to supply the heat energy which is used to expand the solid particles. There is also used liquified petroleum gas or a mixture of propane-butane. The quantity of heat energy required to heat a ton of solid perlite particles to form an expanded perlite particle is in the range of about 3 million to 4.5 million BTU's per ton. It is my understanding that with these three furnaces, viz., the rotary furnace, the horizontal stationary furnace, and the vertical furnace, that the products which can be heat treated are zeolite, vermiculite, and perlite, and products of that class. It is not possible to heat treat diatomaceous earth, clay, cement, fly ash, and titanium dioxide, for example, in the vertical furnace or the stationary horizontal furnace or the rotary furnace.
In these furnaces, it may be considered that two types of air are introduced. One type of air is for combustion purposes so that the fuel, such as a hydrocarbon gas, can be burned to give off heat energy. The second type of air can be considered to be an expansion air. The expansion air, along with the particles to be expanded, is heated to be able to carry away the expanded solid particles or the expanded perlite. Because of the necessity of heating the expansion air, a considerable amount of heat energy is used. The expansion air, at ambient temperature, enters into the furnace, is heated and the temperature elevated to that temperature in the furnace, and this heated expansion air used to carry away the expanded solid products and then the heated expansion air is exhausted to the atmosphere. In one manner of thinking, the heating of the expansion air is a waste of heat energy. As a result of my having worked with these furnaces and having worked in the industry for expanding zeolite, vermiculite, perlite, and the like, I have become familiar with the industry and consider that if a furnace could be devised to eliminate the expansion air, then the heat energy required to make the expanded solid particle would be reduced and there would be a saving in energy. Therefore, I have devised a furnace which can be used for expanding zeolite, vermiculite, perlite, and can process diatomaceous earth, clay, cement, fly ash, titanium dioxide, pumice, and the like, and which furnace uses, essentially, only air for burning the combustible fuel and does not require expansion air for carrying away the expanded solid particles.